1. Field of the Invention
This invention relates to a coloring composition for creating a color filter, enabling the perpendicular (thickness-wise) optical retardation of each of color pixels to be adjusted to become optimal, a color filter wherein the perpendicular optical retardation of each of color pixels is optimized, and a liquid crystal display device which is provided with such a color filter.
2. Description of the Related Art
A liquid crystal display device is a display device wherein the birefringence of liquid crystal molecules is utilized and which is constituted by a liquid crystal cell, a polarizing element and an optical compensating layer. This liquid crystal display device is roughly classified, depending on the kind of light source, into a transmissive type liquid crystal display device, in which the light source is installed inside the device, and a reflection type liquid crystal display device, in which an external light source is utilized.
The transmissive type liquid crystal display device is constructed such that two polarizing elements are mounted on the opposite sides of the liquid crystal cell, and one or two optical compensating layers are interposed between the liquid crystal cell and the polarizing element.
On the other hand, the reflection type liquid crystal display device is constructed such that a reflective plate, a liquid crystal cell, an optical compensation layer and a polarizing element layer are successively arrayed in the mentioned order. The liquid crystal cell is constructed such that orientated bar-like liquid crystalline molecules are sandwiched between two substrates and that as a voltage is applied to electrode layer(s) which is (are) disposed on the opposite sides or one side of the substrates, and the aligned state of bar-like liquid crystalline molecules is caused to change, thereby making it possible to perform the switching of the transmission/shielding of light.
Depending on the alignment of the bar-like liquid crystalline molecules, the liquid crystal cell is permitted to take various display modes, such as TN (Twisted Nematic), IPS (In-Plane Switching), FLC (Ferroelectric Liquid Crystal), OCB (Optically Compensated Bend), STN (Supper Twisted Nematic), VA (Vertically Aligned), and HAN (Hybrid Aligned Nematic).
The polarizing element is generally constructed such that a transparent protective film made of triacetyl cellulose (herein referred to as TAC) is attached to the opposite sides of a polarizing film formed of an oriented polyvinyl alcohol (herein referred to as PVA) in which iodine is diffused.
As for the optical compensating layer, there have been proposed various kinds of layers. For example, in the case of a VA (Vertically Aligned) mode liquid crystal display device which is capable of performing a high-contrast display, there has been employed an optical retardation film exhibiting negative birefringence anisotropy, with the optical axis thereof being perpendicular to the substrate thereof (or negative C plate) together with an optical retardation film exhibiting positive birefringence anisotropy with the optical axis thereof being horizontal to the substrate thereof (or positive A plate) (for example, see JP-A 2000-136253 (KOKAI)).
In recent years, because of the thinness in wall thickness and the resultant advantages such as space-saving, lightweight properties, power-saving, etc., liquid crystal display devices are now rapidly propagated, especially as a display device for televisions, and, consequently, it is required to further enhance various display performance factors, such as the brightness, contrast and omnidirectional visibility.
More specifically, a liquid crystal display device such as an IPS or VA of a normally black mode which makes it possible to realize further enhanced contrast and a wider view field display is employed as especially preferable for use in televisions. With respect to the aforementioned optical compensating layer, many are designed to obtain an optimal value so as to make it possible to minimize the generation of coloring on the occasion of viewing a black color from the front of the television and to minimize the color shift on the occasion of viewing the television obliquely.
There is, however, a problem that, when the values of perpendicular optical retardation of red colored display pixels, green colored display pixels and blue colored display pixels constituting the color filter (hereinafter, referred to as Rth(R), Rth(G) and Rth(B), respectively) differ from each other, coloring is caused to generate on the occasion of viewing a black color obliquely.
Especially, when the values of perpendicular optical retardation of red colored display pixels, green colored display pixels and blue colored display pixels constituting the color filter are non-uniform, i.e. Rth(R)<Rth(G)>Rth(B) or Rth(R)>Rth(G)<Rth(B), it is no longer possible, for the optical compensating layer which is designed to exhibit unidirectional (continuous) wavelength dispersion to the wavelength of light, to compensate the non-uniform values of perpendicular optical retardation among these colors at such a high level in display quality that is demanded nowadays.
More specifically, even though it is possible to realize excellent visibility as the display face is observed from the front side thereof (the direction perpendicular to the display face), when the display face is observed obliquely at an angle of 45 degrees (hereinafter, referred to simply as oblique visibility), only the light of a specific color is caused to leak, resulting in coloring of black color to generate a reddish, bluish or greenish black color.
Since the magnitude of retardation of a color filter is relatively small as compared with that of other components to be employed in a liquid crystal display device, the aforementioned problem was not considered seriously up to date. However, in the case of the liquid crystal television where high contrast and wide viewing-angle properties are demanded, the aforementioned problem cannot be disregarded any longer.
Especially, in the case of the liquid crystal television where a high contrast of not less than 1000 or not less than 3000 is demanded, since the quality of the black color image is required to be excellent, the aforementioned problem cannot be disregarded any longer.
Since the optical designing is now generally performed centering around the green color, if the magnitude of retardation of green display pixels differs greatly from that of red and blue display pixels, light leakage is caused to generate, thus raising problems with respect to the oblique visibility of the display device. With a view to overcome this problem, there has been proposed to incorporate a macromolecule having a planar structure group on its side chain into a colored macromolecular membrane or to incorporate birefringence-reducing particles having a birefringence index which is opposite in sign (positive or negative) to the macromolecule into a colored macromolecular membrane, thereby trying to reduce the magnitude of retardation which the color filter has (for example, see JP-A 2000-136253 (KOKAI) and JP-A 2000-187114 (KOKAI)).
As a matter of fact however, it has been discovered as a result of studies made by the present inventors on this problem, that the value of perpendicular optical retardation of the color filter differs greatly according to the kind of pigment to be employed, the fineness and dispersed state of the pigment, or the kind of matrix resin (for example, acrylic resin or cardo resin). Therefore, it has been found impossible to expect sufficient effects even with the aforementioned methods of incorporating a macromolecule having a planar structure group on its side chain or birefringence-reducing particles into a colored macromolecular membrane, thus failing to solve the aforementioned problem.
Especially, in the case of a color filter wherein a transparent resin, represented by acrylic resin, which enables organic pigments to be readily dispersed therein, is used as a substrate for a high-contrast liquid crystal display device, it has been found difficult to improve the oblique visibility while securing a desired high-contrast value (not less than 1000, more preferably not less than 3000).
Additionally, according to the prior art, it was simplistically believed, erroneously, that an excellent color filter is one with a smaller birefringence, and even though many studies have been made on the means for improving the oblique visibility, no serious study has been made on the means for minimizing the difference in values of perpendicular optical retardation to such a level that does not raise any problem as a high-contrast liquid crystal display device to thereby regulate the perpendicular optical retardation of each color to an optimal value.
Meanwhile, it has been discovered by the present inventors that the retardation of the color filter layer of each of red, green and blue pixel patterns differs depending on the color, i.e. red is enabled to indicate positive or negative retardation, blue is enabled to indicate positive retardation and green is enabled to indicate negative retardation.